Root canal therapy is warranted when bacteria invade the pulp (inner portion that holds the nerve) of the tooth and causes infection. The bacteria usually travels through an opening in the tooth to the pulp, whether it be a deep cavity or a fracture in the enamel both can allow bacteria to invade.
Simply put, a root canal is a dental treatment to rid the pulp of infection and to remove injured/dead pulp tissue as well as the nerve of the tooth. It involves cleaning the pulp canals with small files and sterile irrigating solutions that rid the canals of infection. The clean, hollow canal is then filled with an inert material called gutta percha.
After a root canal, the tooth is at high risk for fracture as the tooth is now hollow as the pulp has been removed. In order to prevent future fracture of a root canaled tooth, the dentist will typically recommend a crown be placed to fully protect it for years to come.
Root canal treatment does not protect your tooth from a new cavity from forming. It is imperative to continue to keep it as clean as possible to prevent further breakdown.
Many times a tooth or teeth can be saved by having root canal therapy but occasionally the damage that is done is irreversible and it is determined by the doctor extraction (removal) is the only option. See Dental Extractions/Bone Grafting.